Chemistry Reference
In-Depth Information
connected by cables and alligator clips with the voltmeter, and the voltage is
measured. (b) The beaker is filled halfway with a concentrated solution of
sodium chloride. Two different metal strips are provided with cables and
alligator clips and connected to the voltmeter; the metal strips are dipped
into the solution and the voltage is measured. The experiment is repeated
with different metal combinations and also with the same metals. The
electric motor is connected as well.
Observation: Dipping into the lemon, potentials of about 1.5 V are measured
between copper and magnesium. In the salt solution the same voltage is
observed, between zinc and magnesium smaller voltage values are
measured. In case of the same metals, no voltage appears. For the higher
voltages, the electric motor does work and shows the running electric
current.
Note: The juice of the lemon is a weak electrolyte solution: the electricity is
not enough to run the electric motor. It succeeds, if two or three lemons are
put in series.
E2.10 Brass Name plate
Problem: Students are motivated more strongly if they produce something
that they can take home. They can produce their own brass name plate as an
introduction into the topic “redox reactions with metals and acids.” This
plate can be used not only at home, but students can also report their families
and friends, how they have produced it: they may feel as experts in this area
of chemistry, and may be proud of their competence.
Material: Glass bowl, beaker (100 mL), pipette, brass plate or copper plate,
candle, iron nail, knife; pure nitric acid (C), boiling chips, gasoline (Xn/F/N),
filter paper.
Procedure: One side of the metal plate is coated with heated liquid paraffin
from the candle. With the iron nail you scratch a name or a desired figure
deep into this layer of paraffin, so that the metal is exposed. You put the plate
with the top layer of wax on a few boiling chips into the glass bowl and put it
under the exhaust hood. In the beaker 5 mL of water and 10 mL of pure nitric
acid are mixed and the mixture is dripped with the pipette onto the scratched
parts of the paraffin layer. You wait about 10 min.
Observation: The acidic solution reacts with the metal by forming a brown
gas (nitric oxide) and a blue solution (copper nitrate solution). After removal
of the remaining paraffin with the knife and gasoline-wet filter paper, the
name or label is clearly visible in the metal plate.
Disposal: The name plate and the glass bowl is rinsed with much water, the
gasoline-wet filter paper is to be burnt under the exhaust hood.
M2.1 Sphere-packing model of a salt crystal
Problem: The manual production of structural models is highly motivating
for many students, especially if they are allowed to take these models home.
It is easily possible to illustrate the topic “composition of salts” with the
example of
the sodium chloride crystal structure,
for example,
the
